1. Field of the Invention
The present invention relates to an apparatus for determining an activated condition of an oxygen sensor.
2. Description of the Related Art
In an internal combustion engine for obtaining a desired air-fuel ratio, an oxygen sensor, such as a lean sensor, is used for obtaining an output signal which corresponds to the density of the oxygen of the exhaust gas from the engine. The output signal from the oxygen sensor controls a duration of a time of the opening of the fuel injector, to obtain an air-fuel ratio of the internal combustion engine set to a value larger than the stoichiometric air-fuel ratio.
A known oxygen sensor includes a body of a solid electrolyte, such as a stabilized zirconia, a first air permeable electrode on one side of the solid electrolyte body and in contact with the exhaust gas to be detected, and a second air permeable electrode on the other side of the solid electrolyte body and in contact with the reference gas, such as atmospheric air.
In this type of sensor, an application of a direct current electric voltage within a predetermined range across the electrodes obtains an electric current, called a limit electric current, which is maintained at a predetermined value in accordance with a density of the oxygen in the exhaust gas. Therefore, the detection of the output electric current from the oxygen sensor enables a value of the density of the exhaust gas to be obtained, and using this value, it is possible to estimate the value at which the air-fuel ratio for the internal combustion engine should be set.
Nevertheless, a low temperature of the solid electrolyte body (i.e., element temperature) in this type of oxygen sensor causes the sensor output value to be reduced while maintaining the same air-fuel ratio within a range at which it is used. Namely, the electric current value obtained corresponds to an oxygen density value which is lower than the actual value of the oxygen density, and as a result, a feed-back control of the air-fuel ratio in accordance with the output signal from the sensor during a low temperature state of the element causes the air-fuel ratio to be incorrectly controlled to a value larger than the preset air-fuel ratio.
One solution to such a problem is to provide a means of determining if a state is reached wherein the oxygen sensor is "fully activated", before executing the feed-back control of the air-fuel ratio. A prior art method of obtaining such a determination of the activated condition of the oxygen sensor uses the output level from the oxygen sensor when the engine is under a fuel cut (F/C) condition. This technique employs a principle such that the oxygen density of the exhaust gas during the fuel cut operation is equal to the oxygen density of the atmospheric air, and a determination of whether or not the oxygen sensor is in an activated state is obtained by determining if the output level from the oxygen sensor corresponds to the oxygen density of the atmospheric air. (See Japanese Unexamined Patent Publication No. 59-46350.) In this prior art, a threshold level is provided and it is determined whether the output level from the oxygen sensor has become higher than the threshold level during the fuel cut condition. When the output level from the oxygen sensor is higher than the threshold level, it is determined that the element temperature is high enough to ensure that the oxygen sensor is in an activated condition.
To improve the precision of the determination of the activated state of the oxygen sensor, a technique has been proposed whereby it is determined whether a condition, wherein the output level from the oxygen sensor is higher than the threshold value during the fuel cut condition, is continued. (See Japanese Unexamined Patent Publication No. 60-21265.) In this prior art, a measurement of the output of the oxygen sensor is repeatedly carried out at predetermined intervals during the fuel cut condition, and a determination of the activated state of the oxygen sensor is obtained when a continuation of a state wherein the output level of the oxygen sensor is higher than the threshold value for longer than a predetermined interval is determined.
In this prior art, the reason for the determination of the continuation of the state wherein the output level of the oxygen sensor is higher than the threshold value for longer than the predetermined interval, is to obtain a determination of the activated condition of the oxygen sensor as quickly as possible without losing the precision of the determination. The threshold value for determining the activation condition is usually the lowest possible value for obtaining a quick determination of the activated condition of the element, and thus an incorrect determination is apt to be made, since inevitably some fluctuation of the output level from the oxygen sensor will occur, which causes a situation to arise such that the output level from the oxygen is higher than the threshold value even if the sensor is not actually activated. Such an incorrect determination due to the fluctuation of the output level is avoided by determining if the sensor output level remains higher than the fixed value for a time longer than a predetermined interval.
The above-mentioned prior art suffers from a drawback in that a proper determination of the activated condition of the oxygen sensor cannot be always obtained, for the following reason. Where the engine is in a cruising state, e.g., when running on a freeway, if a steady state condition continues for a relatively long time, a fuel cut operation occurs only rarely. In this case, the prior art method of determining the activated state of the oxygen sensor by the detection of duration of the state where the sensor level is higher than a predetermined level for longer than the predetermined interval during the fuel cut operation, may sometimes cause the air-fuel ratio feedback control to be stopped for a relatively long time, and thus a precise control of the air-fuel ratio cannot be obtained.